Engineered biosorbents of pomelo (Citrus maxima (Burm.f.) Merr) peels modified with zinc oxide and titanium dioxide for methylene blue dye sorption

The pomelo-doped zinc oxide beads (PZB), pomelo-doped titanium dioxide beads (PTB), and pomelo-doped zinc oxide and titanium dioxide beads (PZTB) were synthesized for sorbing methylene blue (MB) dye. Their characterizations were explored by X-Ray Diffractometer (XRD), Field Emission Scanning Electron Microscopy and Focus Ion Beam (FESEM-FIB), Energy Dispersive X-Ray Spectrometer (EDX), and Fourier Transform Infrared Spectroscopy (FT-IR). In addition, their sorbent efficiencies for sorbing MB dye were investigated through batch experiments, sorbent reusability studies, sorption isotherms, kinetics, and thermodynamic studies. They were crystalline phases presenting the specific peaks of zinc oxide (ZnO) or titanium dioxide (TiO2). Their surfaces had lamella structures with coarse surfaces, and they also found specific structures of ZnO or TiO2 on the surfaces. Zn–O or Ti–O–Ti was also detected in PZB or PTB or, PZTB depending upon metal oxide types added into pomelo beaded sorbents. For batch experiments, they could adsorb MB dye of more than 86%, and PZTB showed the highest MB dye removal efficiency. In addition, they could be reused for more than three cycles with high MB dye sorptions of more than 72%. They corresponded to Freundlich and pseudo-second-order kinetic models. Moreover, the increasing temperature affected their decreasing MB dye sorptions which were exothermic processes.


Sorbent synthesis
The sorbent synthesis of pomelo-doped zinc oxide beads (PZB), pomelo-doped titanium dioxide beads (PTB), and pomelo-doped zinc oxide and titanium dioxide beads (PZTB) was demonstrated in the schematic flow diagram in Fig. 1.Firstly, 10 g of PP was added to the ZnO or TiO 2 , or ZnO + TiO 2 solution which was prepared from 8 g of ZnO or 8 g of TiO 2 , or 4 g of ZnO + 4 g of TiO 2 in 160 mL of deionized (DI) water.Then, it was mixed by an orbital shaker (GEL, 3020, Germany) of 200 rpm for 3 h, filtrated, and air-dried called pomelo powder doped ZnO (PPZ) or pomelo powder doped TiO 2 (PPT), or pomelo powder doped ZnO and TiO 2 (PPZT).After that, it was added to the sodium alginate solution which was prepared from 8 g of NaC 6 H 7 O 6 dissolved in 400 mL of DI water.It was homogeneously mixed by a hot plate (Ingenieurbüro CAT, M. Zipperer GmbH, M 6, Germany) with a magnetic stirrer of 200 rpm at 60 °C.Then, it was contained in a 10 mL syringe with a needle of 1.2 × 25 mm and was added dropwise into 0.1 M CaCl 2 .2H 2 O solution.After that, they were filtrated, washed with DI water, air-dried for 24 h, and kept in a desiccator before use called pomelo-doped zinc oxide beads (PZB) or pomelo-doped titanium dioxide beads (PTB), or pomelo-doped zinc oxide and titanium dioxide beads (PZTB).

Sorbent characterizations
The characterizations of PZB, PTB, and PZTB on crystalline structures by X-Ray Diffractometer (XRD) analysis at 2ϴ of 5-80°, sorbent structures and chemical elements by Field Emission Scanning Electron Microscopy and Focus Ion Beam (FESEM-FIB) with Energy Dispersive X-Ray Spectrometer (EDX) by using 10 kV measurement which the samples were coated with gold, and chemical functional groups by Fourier Transform Infrared Spectroscopy (FT-IR) from 4000 to 600 cm −1 with a resolution of 4 cm −1 and 32 scans are investigated.

The point of zero charge
The point of zero charge method is referred from the study of Praipipat et al. 19 to determine what pH value represents its zero charge which might occur in MB dye sorption by each sorbent.In summary, 0.1 g of PZB or PTB, or PZTB was added to 50 mL of 0.1 M NaCl solution at pH 1-12 and shaken by an orbital shaker at 150 rpm for 24 h.Then, the final pH was measured by a pH meter (Mettler Toledo, Seven2Go pH with InLab Expert Go-ISM, Switzerland), and the intersection point between ∆pH (pH final -pH initial ) and pH presented pH pzc .where C 0 is the initial MB dye concentration (mg/L), and C e is the final MB dye concentration (mg/L).
where V is the sample volume (L), and M is the mass of sorbent (g).

Sorbent reusability studies
The method of sorbent reusability studies is referred from the study of Praipipat et al. 20 to investigate the sorbent stability with reuse through the desorption experiments.In summary, 0.5 M HCl was used as a desorption agent to push out MB dye ions from the sorbent by adding the saturated PZB or PTB, or PZTB into 100 mL of 0.5 M HCl solution and shaking by an orbital shaker of 150 rpm for 2 h.After that, they were filtrated, rinsed with DI water, and air-dried, then they were ready for the next adsorption cycle.Equation (3) was used to calculate the desorption efficiency in the percentage.
where q d is the amount of MB dye desorbed (mg/L) and q a is the amount of MB dye sorbed (mg/L).
where q e is the capacity of dye sorption on sorbent at equilibrium (mg/g), q m is the maximum amount of dye sorption on sorbent (mg/g), C e is the equilibrium of dye concentration (mg/L), and K L is Langmuir sorption constant (L/mg).
where K F is the Freundlich constant of sorption capacity (mg/g)(L/mg) 1/n and n is the constant depicting the sorption intensity.
where R is the universal gas constant (8.314J/mol), T is the absolute temperature (K), b T is the constant related to the heat of sorption (J/mol), and A T is the equilibrium binding constant corresponding to maximum binding energy (L/mg).
where K DR is the activity coefficient related to mean sorption energy (mol 2 /J 2 ), and ε is the Polanyi potential (J/ mol).
where q t is the amount of sorbed dye at the time (t) (mg/g).k 1 is pseudo-first-order and pseudo-second-order rate constant (1/min).
where k 2 is pseudo-second-order rate constant (g/mg•min).
where α is the initial sorption rate (mg/g•min) and β is the extent of surface coverage (g/mg).
(1) MB dye removal efficiency(%) where k i is the intraparticle diffusion rate constant (mg/g•min 0.5 ) and C i is the constant that gives an idea about the thickness of the boundary layer (mg/g).
For sorption isotherms, 0.1 g of PZB or 0.1 g of PTB, or 0.08 g of PZTB was added into 250 mL of Erlenmeyer flask containing 100 mL of different MB dye concentrations from 25 to 200 mg/L, adjusted at pH 7, and tested the MB dye sorption by an incubator shaker at 25 °C for PZTB or 30 °C for PZB and PTB with a shaking speed of 150 rpm for 12 h.
For sorption kinetics, 1 g of PZB or 1 g of PTB, or 0.8 g of PZTB was added into 1000 mL of breaker containing 1000 mL of 100 mg/L of MB dye concentration, adjusted at pH 7, and tested the MB dye sorption by a jar test (JAR-TESTER, SF6, South Korea) with a shaking speed of 150 rpm for 15 h.

Thermodynamic studies
The changing temperature might affect MB dye sorptions of PZB, PTB, and PZTB, so the thermodynamic studies were designed to examine how much it affects by using Eqs.( 12), ( 13), ( 14) to calculate thermodynamic parameters 30 .
where R is the universal gas constant (8.314J/mol K), T is the absolute temperature (K), and K c is the equilibrium constant (L/mg).
where ∆H° and ∆S° values were calculated from the slope and intercept of the linear graph between ln K c (K c = q e /C e ) and 1/T.
For the thermodynamic study, 0.1 g of PZB or 0.1 g of PTB, or 0.08 g of PZTB were added to 250 mL of Erlenmeyer flask containing 100 mL of 100 mg/L MB dye concentration, adjusted at pH 7, and tested the MB dye sorption by an incubator shaker with different temperatures from 293.15 to 323.15 K with a shaking speed of 150 rpm at 12 h.

The physical characteristics
The physical characteristic images of PZB, PTB, and PZTB were demonstrated in Fig. 2a-c which had a spherical shape.They had different colors depending on the type of metal oxide added to pomelo beaded sorbents.PZB had cream-colored beads, whereas PTB had white-colored beads.For PZTB, it had light cream color beads which was a color between PZB and PTB.

Field Emission Scanning Electron Microscopy and Focus Ion Beam (FESEM-FIB)
The FESEM-FIB images revealed the surface morphologies of PZB, PTB, and PZTB at 120X magnification with 1 mm, 3500X magnification with 50 µm, and 25,000X magnification with 5 µm for the spherical shape, surface by a cross-section, and metal oxide zooming by a cross-section, respectively shown in Fig. 4a-i.For a spherical shape, they were heterogeneous surfaces shown in Fig. 4a-c.For the surface, they had lamella structures with coarse surfaces that corresponded to the pomelo peel structure found in a previous study 11 demonstrated in Fig. 4d-f.For metal oxide zooming, zinc oxide (ZnO) was found in PZB which had a square shape similar found in a previous study 33 shown in Fig. 4g.While titanium dioxide (TiO 2 ) was found in PTB which had a circular shape similar reported by a previous study 34 illustrated Fig. 4h.For PZTB, the distribution of ZnO and TiO 2 was found on its surface demonstrated in Fig. 4i.
Energy Dispersive X-Ray Spectrometer (EDX) PZB, PTB, and PZTB had five main chemical elements of carbon (C), oxygen (O), calcium (Ca), sodium (Na), and chloride (Cl), whereas zinc (Zn) or titanium (Ti) also detected them with close percentage by weight of 1:1 following a ratio metal oxide added in the synthesis method shown in Table 2.Moreover, their elemental mapping distributions were illustrated in Fig. 5a-c with the main elements mentioned above distributed on their surfaces.In addition, Zn and Ti were also distributed throughout their surfaces confirming the addition of two metal oxides into pomelo beaded sorbents.

Fourier Transform Infrared Spectroscopy (FT-IR)
Figure 6a-c displayed the chemical functional groups of PZB, PTB, and PZTB which had four main functional groups consisting of hydroxyl (O-H), methyl and methylene (C-H), carbonyl (C=O) or (C-O-C), and carboxyl (C-O).Furthermore, the specific peaks of zinc oxide (ZnO) or titanium dioxide (TiO 2 ) were also demonstrated in pomelo beaded sorbents.In the wavenumbers of 3200 and 1300 cm −1 , they represented O-H of pectin, cellulose, lignin, carboxylic acids, and phenol compounds 11 .For the wavenumber of 2900 cm −1 , it displayed C-H in methyl (CH 3 ) and methylene (CH 2 ) groups 35 .The wavenumber of 1500 cm −1 referred to C=O of carboxylic acids in lignin and sodium alginate 36 .C-O presented ionic carboxylic groups found in the wavenumber of 1500 cm −137 .For C-O-C, it showed the sodium alginate and anhydroglucose of cellulose or hemicellulose found in the wavenumber of 1000 cm −136 .Moreover, the specific peaks of zinc oxide found in PZB and PZTB displayed in the form of Zn-O in wavenumber of 627 and 629 cm −1 , and the specific peaks of titanium dioxide found in PTB and PZTB shown in the form of Ti-O-Ti in wavenumber of 626 and 629 cm −12 .The pomelo peels and sodium alginate were formed through an ion exchange process 38 , and then their hydroxyl groups (O-H) interacted with the hydroxyl groups (O-H) in ZnO or TiO 2 through hydrogen bonds to become composited sorbents (Zn-O/Ti-O-Ti-C=O-OH) 39 .After MB dye sorptions, the intensity peaks in their chemical functional groups especially O-H and C=O positions were decreased with a little bit of shift of wavelengths.In addition, the low-frequency bands of Zn-O and Ti-O-Ti were detected.These might be evidence of their MB dye sorptions similar reported in the previous study 40 .

The point of zero charge (pH pzc )
Figure 7a-c illustrated the results of the points of zero charge of PZB, PTB, and PZTB which were 7.19, 5.36, and 7.04, respectively.The addition of zinc oxide into pomelo beaded sorbents had a higher pH pzc than titanium dioxide, whereas the addition of both metal oxides had a middle pH pzc between each other.As a result, their MB dye sorptions should occur at pH solution more than their pH pzc .

Batch experiments
The sorbent efficiencies for MB dye sorptions of PZB, PTB, and PZTB were investigated by batch experiments, and the results were illustrated in Fig. 8a-e.For the effect of dosage, the increasing dosage resulted in the increases of their MB dye removal efficiencies from 0.02 to 0.1 g, and then they were constant.This might be from the increasing dosage is the increase of active site, so the sorbent can be more sorbed dye ions 41 .However, the highest dye removal efficiency does not exceed the equilibrium sorption of sorbent which observes the dye removal efficiency to be constant 19,31 .In addition, the highest MB dye removal efficiencies of PZB, PTB, and PZTB were found at 0.1 g (86.33%), 0.1 g (89.15%), and 0.08 g (92.52%), respectively.For the effect of contact time, the increasing contact times affected their increasing MB dye removal efficiencies from 3 to 12 h because the sorbents had a long time to sorb MB dye.After 12 h, their MB dye removal efficiencies were constant because they had gone into the equilibrium sorptions related to the saturation of their active sites 42 .Thus, the contact time of 12 h represented the equilibrium contact time of all sorbents because it demonstrated their highest MB dye removal efficiencies of PZB, PTB, and PZTB at 12 h for 85.55%, 88.33%, and 91.60%, respectively.For the effect of temperature, the increasing temperature resulted in the increase of MB dye removal efficiencies from 20 to 30 °C, and then they decreased.As a result, they did not favor MB dye sorptions at high temperatures caused by the changing of sorbent structures such as cellulose, and lignin 43 .Moreover, the highest MB dye removal  efficiencies of PZB, PTB, and PZTB were found at 30 °C (84.63%), 30 °C (88.37%), and 25 °C (92.07%), respectively.For the effect of pH, the increasing pH resulted in the increase of MB dye removal efficiencies from pH 5-7, and then they were constant.At pH 5-6, the surfaces of PZB, PTB, and PZTB were positively charged, so they had low MB dye removal efficiencies because of the force of repulsion between charges of the same type.At pH 7-12, their surfaces were negatively charged, so they had high MB dye removal efficiencies.Furthermore, the highest MB dye removal efficiencies of PZB, PTB, and PZTB were found at pH 7 for 84.29%, 87.88%, and 91.63%, respectively which corresponded with their MB dye sorptions should occur at pH solution > pH pzc .For the effect of concentration, the increasing concentration resulted in decreasing MB dye removal efficiencies because of the decrease of the active site of sorbents.Therefore, the optimum conditions of dosage, contact time, temperature, pH, and concentration of PZB, PTB, and PZTB were 0.1 g, 12 h, 30 °C, pH 7, and 100 mg/L for 86.75%, 0.1 g, 12 h, 30 °C, pH 7, and 100 mg/L for 89.26%, and 0.08 g, 12 h, 25 °C, pH 7, and 100 mg/L for 92.11%, respectively.Moreover, they could be arranged from high to low MB dye removal efficiency to be PZTB > PTB > PZB, and PZTB illustrated the highest MB dye removal efficiency than other sorbents with not only offering high MB dye removal efficiency but also spending less dosage and temperature than others.Therefore, the pomelo beaded sorbents with ZnO or TiO 2 modifications increased sorbent efficiency especially the mixture of ZnO and TiO 2 in a ratio of 1:1.

Sorbents reusability studies
The sorbent reusability was studied to investigate how many cycles PZB, PTB, and PZTB were potential MB dye sorptions through the desorption experiments using three sorption-desorption cycles to estimate the cost-effectiveness of sorbents before use in industrial wastewater treatment.The results were illustrated in Fig. 9a-c, and they could be adsorbed with MB dye for more than three cycles with more than 72%.For PZB, www.nature.com/scientificreports/ the sorption-desorption in three cycles were in ranges of 72.34-84.17%and 79.96-98.21%which were decreased by approximately 12% and 18%.For PTB, the sorption-desorption in three cycles were in ranges of 77.12-87.61%and 82.51-99.54%which were decreased by approximately 10% and 17%.For PZTB, the sorption-desorption in three cycles were in ranges of 84.58-90.82%and 85.29-100% which were decreased by approximately 6% and 15%.Therefore, pomelo beaded sorbents were good for application in industrial wastewater treatment offering cost-effective investment.

Sorption isotherms
The results of sorption isotherms of PZB, PTB, and PZTB were displayed in Fig. 10a-c, and Table 3 illustrated their isotherm parameters.Since the highest R 2 value is a good fit model to describe the sorption pattern, the Freundlich model was chosen because it presented the highest R 2 close to 1 in all sorbents compared to other isotherm models.As a result, their sorption patterns are related to physiochemical sorption which might be from the addition of zinc oxide or titanium dioxide into pomelo beaded sorbents similar reported by other studies 2,21,44 .For 1/n and K F comparison, since their 1/n values were in a range of 0 < 1/n < 1, they favorably sorbed MB dye.Moreover, PZTB had a higher Freundlich constant of sorption capacity (K F ) than other sorbents, so it was possible more MB dye sorption than PZB and PTB.The comparison of the maximum sorption capacity (q m ) of pomelo sorbents with or without modifications of thermal, chemical, and metal for MB dye sorptions is demonstrated in Table 4 which PZTB had a higher q m than others.While PZB and PTB had higher q m than pomelo peel biochar at 500 °C and 600 °C and pomelo peel modified by iron(III) chloride.Therefore, they were potential beaded sorbents for removing MB dye in an aqueous solution, especially PZTB.

Sorption kinetics
The results of the sorption kinetics of PZB, PTB, and PZTB were demonstrated in Fig. 11a-c and Table 5 displayed their kinetic parameters.Since the R 2 values of the pseudo-second-kinetic model were higher than the R 2 values of other kinetic models with close to 1, their rates and mechanisms were well described by the pseudo-secondkinetic model relating to a chemical sorption process similar to other studies reported 2,21,44 .Moreover, RMSE is also another value used for deciding the good-fit kinetic model, and the low RMSE is chosen.In Table 6, since the RMSE in the pseudo-second-kinetic model demonstrated the lowest value in all sorbents than other models, their rates and mechanisms corresponded with the pseudo-second-kinetic model agreed with the results of R 2 .
For q e and k 2 comparisons, PZTB had a higher q e than other sorbents, so it could have more ability to sorb MB dye than others.While the k 2 value of PTB was higher than other sorbents, it had a higher sorption MB dye rate than others.

Thermodynamic studies
The thermodynamic study is used for investigating how much temperature affects MB dye sorption by sorbent and to know whether its sorption process can naturally occur.Table 6 and Fig. 12a-c demonstrated their thermodynamic results.Since their ∆G° values were negative, their MB dye sorptions could naturally occur.In addition, their sorption processes were exothermic processes they did not favor MB dye sorptions with increasing temperature because of their negative ∆H° values.Moreover, the ∆H° value can also indicate the sorption process that the sorbent is holding.If ∆H° is in the range of (− 20)-(0) kJ/mol, it is a process of physic sorption.If ∆H° is in the range of (− 80)-(− 400) kJ/mol, it is a process of chemical sorption 45,46 .Since the ∆H° values of PZB, PTB, and PZTB were in the range (− 20)-(− 80) kJ/mol, they were both physical and chemical sorption processes.For ∆S˚, they were also negative which meant their sorption processes randomly decreased in solid solution interfaces.

The feasible mechanisms of MB dye sorptions
Both physisorption and chemisorption played important roles in describing feasible multi-mechanisms of MB dye sorptions by PZB, PTB, and PTZB modified an idea from Praipipat et al. 44 .Multi-mechanisms included the electrostatic attraction, hydrogen bonding interaction, n-π interaction, and complexation were demonstrated in Fig. 13.For electrostatic attraction, the negative charges of PZB, PTB, and PTZB at pH solution > their pH pzc interacted with MB dye ions 11 .For hydrogen bonding interaction, the hydroxyl groups (O-H) on their surfaces caught the nitrogen atom in the MB dye structure 47 .For n-π interaction, the carbonyl (C=O) or (C-O-C) on their surfaces interacted with the aromatic rings of MB dye 11 .For the complexation, O-H, C=O, Zn-O, and Ti-O-Ti functional groups in composited sorbents of PZB, PTB, and PZTB could catch the positively charged MB dye molecules through chemical affinity 48 .

Conclusion
This study presented the performances of three pomelo beaded sorbents which were pomelo-doped zinc oxide beads (PZB), pomelo-doped titanium dioxide beads (PTB), and pomelo-doped zinc oxide and titanium dioxide beads (PZTB) to eliminate MB dye in an aqueous solution.The successful additions of zinc oxide (ZnO) or titanium dioxide (TiO 2 ) into PZB, PTB, and PZTB were confirmed by XRD, EDX, and FT-IR techniques which presented the specific peaks of ZnO or TiO 2 , chemical elements of Zn or Ti, and chemical functional groups of Zn-O or Ti-O-Ti.Moreover, their MB dye removal efficiencies could be arranged from high to low of PZTB > PTB > PZB.Since PZTB demonstrated the highest MB dye removal efficiency of 92.11%, the additions of both ZnO and TiO 2 were recommended to increase the capacity of pomelo beaded sorbents.Furthermore, their sorption patterns and mechanisms corresponded to Freundlich and pseudo-second-order kinetic models, and they could be reused for more than three cycles.Their MB dye sorption mechanisms could be explained by multi-interactions of electrostatic attraction, hydrogen bonding interaction, n-π interaction, and complexation.Therefore, they are potential sorbents for sorbing MB dye in wastewater, especially PZTB.Before industrial applications, the column experiments with continuing flow rate should be investigated, and the competing ions are interesting investigations to confirm the specific target sorption of PZB, PTB, and PZTB.

Time (min) Temp. (°C) pH Conc. (mg/L) Volume (mL) q m (mg/g) References
MB dye removal efficiencies of PZB, PTB, and PZTB were investigated by batch tests on dose (0.02-0.5 g), contact time (1-12 h), temperature (20-50 °C), pH(5-12), concentration (25-200 mg/L) with the control conditions of MB dye concentration of 100 mg/L, shaking speed of 150 rpm, and sample volume of 100 mL to determine their optimum conditions obtained the highest MB dye removal efficiency.The triplicate samples were applied to verify the results.MB dye concentration was measured by a UV-vis spectrophotometer (Specord 200 Plus Double-beam, Analytikjena, Germany), and the MB dye removal efficiency in percentage and the sorption capacity were calculated following Eqs.(1) and (2).

Table 3 .
The isotherm parameters for MB dye sorptions on PZB, PTB, and PZTB.

Table 4 .
Comparison of the maximum sorption capacity (q m ) of pomelo sorbents for MB dye sorption.

Table 5 .
The kinetic parameters of PZB, PTB, and PZTB for MB dye sorptions.